<!DOCTYPE html>
<html lang="en">

<head>
    <meta charset="UTF-8">
    <meta http-equiv="X-UA-Compatible" content="IE=edge">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
    <script>
        var num1 = + prompt('请输入第一个值');
        var num2 = + prompt('请输入第二个值');
        var num3 = + prompt('请输入第三个值');
        var max = ((num1 > num2 && num1 > num3) ? num1 : num2) > num3 ? ((num1 > num2 && num1 > num3) ? num1 : num2) : num3;
        alert('最大值是' + max);
    </script>
</head>

<body>

</body>

</html>